When you create a public holiday you can assign employees to them at any time.
Click your name in the top right, then click Settings.
On the settings menu click Calendar, then click Public holiday groups.
On a public holiday group, click on the Eligibility button.
Select the employee(s) you want to assign to this public holiday group. You can search for them or click on the cogs icon for an advanced search, where you can filter down to employees of a certain Team, Position and/or Location.
Once selected click Save.